It is not possible to downgrade the version. Technically, you would need to downgrade it to 1.12.1, then use the tutorial panoramix made to upgrade to 1.12.3.
So, i think the easyest way is to reinstall a new version and try to see if it is possible to upgrade to 1.12.1 only without the rest
If this is not possible, the last solution would be to try to get the whole file from someone and just copy paste from comp to comp... This is kind of a hardcore and unclean solution, but it works.
! My Life for The HORDE ! Victory For Sylvanas !